Defective intestinal epithelial tight junction (TJ) barrier is a key pathogenic factor of inflammatory bowel disease (IBD) and other inflammatory conditions of the gut. A leaky TJ barrier allows increased intestinal permeation of bacterial antigens that induce inflammatory response. Previous studies have shown that tightening (or re-tightening) of the intestinal TJ barrier prevents the development of intestinal inflammation in both animal models of IBD and in human IBD. However, there are no currently available therapeutic agents that target the intestinal TJ barrier. There is also an important gap in medical knowledge regarding the intracellular processes that can be targeted to induce therapeutic tightening of the intestinal TJ barrier. In this grant application, we intend to 1) introduce a new therapeutic agent that targets the intestinal TJ barrier which can be rapidly advanced for clinical usage; and 2) identify intracellular mechanisms that can be targeted to induce tightening of the intestinal TJ barrier. In our preliminary studies, we tested number of probiotic species/strains that are widely available commercially to identify a single strain, Bifidobacterium bifidum VIII-21 (BB), which causes a marked enhancement in intestinal TJ barrier and has therapeutic efficacy in animal models of IBD. The over-arching goals of this application are to investigate the intestinal TJ barrier augmenting effects of BB and to determine the therapeutic efficacy of BB in animal models of IBD. Based on our compelling preliminary data, we advance a novel hypothesis that BB protects against the development of intestinal inflammation by enhancing and preserving the intestinal TJ barrier; and that BB enhancement and protection of intestinal TJ barrier is mediated by Nod1 signal-transduction pathway activation of occludin gene and suppression of myosin light chain kinase (MLCK) gene. In this grant application, we also challenge 2 well-established scientific paradigms: 1) that Nod1 is a cytoplasmic pattern recognition receptor (PRR); and 2) that the primary cellular target of Nod1 is the activation of NF-?B. We also hypothesize that BB protects against pro-inflammatory cytokine-induced increase in intestinal TJ permeability by Nod1/PPAR- ? mediated suppression (not activation) of NF-?B and MLCK gene. Four inter-linked specific aims are proposed to address above hypotheses: 1) to delineate the role of PRRs in BB-induced augmentation of intestinal epithelial TJ barrier; 2) to delineate the molecular mechanism of BB modulation of intestinal TJ barrier; 3) to delineate the protective mechanism of BB against of pro-inflammatory cytokine-induced increase in intestinal TJ permeability; and 4) to delineate the therapeutic efficacy of BB in anima models of IBD. The successful completion of the proposed studies will help bridge the important gap in scientific knowledge and provide crucial pre-clinical data to support the planned future clinical studies.